The influence of cyanide on the carbonylation of Iron(II): Synthesis of Fe-SR-CN-CO centers related to the hydrogenase active sites

Full text of DOI:10.1021/ja015948n

J. Am. Chem. Soc. 2001, 123, 6933-6934
The Influence of Cyanide on the Carbonylation of
6933
Iron(II): Synthesis of Fe-SR-CN-CO Centers
Related to the Hydrogenase Active Sites
Thomas B. Rauchfuss,* Stephen M. Contakes,
Sodio C. N. Hsu, Michael A. Reynolds, and Scott R. Wilson
Department of Chemistry
UniVersity of Illinois at Urbana-Champaign
Urbana, Illinois 61801
Figure 1. Structure of the dianion in (Et4N)2[Fe(SC6H5)2(CN)2(CO)2]
(1a) with thermal ellipsoids set at the 50% probability level. Selected
distances (Å) and angles (deg): Fe-C1, 1.935(4); Fe-C2, 1.928(4); Fe-
C3, 1.782(4); Fe-C4, 1.805(4); Fe-S1, 2.3479(10); Fe-S2, 2.3489(10);
S(1)-Fe(1)-S(2), 82.43(3).
ReceiVed April 4, 2001
ReVised Manuscript ReceiVed May 30, 2001
Recently iron sulfides have been proposed as being central to
the emergence of life.1 For example, Huber and Wa¨chtersha¨user
showed that iron sulfides catalyze carbonylation reactions leading
to the formation of peptides and thioesters.2 The two best
characterized Fe-S-CO enzymes (the hydrogenases) also feature
cyanide, and it is likely that cyanide has a decisive stabilizing
effect on the CO binding. Cyanide has been previously considered
in the prebiotic context,3 but the influence of cyanide on the
carbonyl chemistry of iron has received scant attention. In this
report, we show that cyanide has a major effect on the carbon-
ylation of ferrous salts, especially in the presence of sulfur
ligands.
Scheme 1
CO-saturated MeCN slurries of FeCl2 were treated sequentially
with NaSAr and Et4NCN to give good yields of (Et4N)2[Fe(SAr)2-
(CN)2(CO)2] (1a, Ar ) Ph; 1b, Ar ) p-tol) (Scheme 1). These
same species also form in low yield upon treatment of Fe3(SPh)6-
(CO)6 with CN-.4 NMR and IR5 spectroscopic studies established
that 1a and 1b exist in solution as both the trans,cis- and cis,-
cis-isomers. The molecular structure of trans,cis-1a was deter-
mined crystallographically (Figure 1). In solution 1 is configu-
rationally stable under a CO atmosphere, although in the absence
of CO it suffers ligand redistribution to give (Et4N)2[Fe(SPh)4]
and trans-[Fe(CN)4(CO)2]2- (2) (vide infra). Marko´ had previously
demonstrated the carbonylation of Fe(II) thiolate solutions in the
presence of chelating donor ligands, e.g. bipyridine, ethylenedi-
amine, and Ph2PCH2CH2PPh2.6
valent iron may be significant in view of the likely role of
subferrous species in the iron-only hydrogenases.9 Interestingly,
the reductive nature of this carbonylation is quenched by the
presence of cyanide. We showed that Fe2(SEt)2(CO)6 reacts with
CN- to give [Fe2(SEt)2(CN)2(CO)4]2-, but such subferrous species
are not observed when the Fe(II)/NaSEt solutions are carbonylated
in the presence of CN- (Scheme 1),10 i.e., our studies do not
support the spontaneous assembly of hydrogenase-like subferrous
species from Fe(II)/CN- solutions.
In analogy to the preparation of 1, we examined the carbon-
ylation of Fe(II) solutions in the presence of benzenedithiolate
dianion (bdt2- ) C6H4S22-). This reaction afforded complexes
(Et4N)2[Fe(bdt)(CN)2(CO)] (3) and (Et4N)2[Fe(bdt)(CN)2(CO)2]
(4). Initially complex 4 is observed spectroscopically; however,
purging N2 through the reaction solution gave 3, which was
characterized crystallographically as being pentacoordinate (Figure
2). The Fe-CN and Fe-CO distances differ by 0.2 Å, consistent
with the strong π-bonding role of the CO vs the primary
σ-interaction for the CN- ligand. Several related 16 e- pentacoor-
dinate Fe(II) dithiolenes are known, e.g., Fe(bdt)(PMe3)3 and Fe-
[S2C2(SMe)2](CO)(PR3)2,11 but 3 is distinctive because it very
closely simulates the Fe site in the [NiFe]-hydrogenases, which
also feature (SR)2(CN)2(CO) coordination.12
Carbonylation of nonaqueous Fe(II)/EtS- solutions (as de-
scribed for the preparation of 1) in the presence of CN- afforded
2, not analogues of 1. IR spectra of fresh reaction solutions
indicate that [Fe(SEt)2(CN)2(CO)2]2- is in fact formed, but that
this species redistributes readily to the tetracyanide.
In the absence of CN-, carbonylation of Fe(II)/PhS- solutions
gives the ferrous derivatives Fe3(SPh)6(CO)6 and [Fe(SPh)3-
(CO)3]-.4,6,7 The carbonylation of Fe(II)/NaSEt solutions affords
the subferrous species Fe2(SEt)2(CO)6 as first reported by
Reihlen.8 The yields are low (3-6%), and pyrrophoric iron metal
is also formed in substantial amounts, but the formation of low
(1) Cody, G. D.; Boctor, N. Z.; Filley, T. R.; Hazen, R. M.; Scott, J. H.;
Sharma, A.; Yoder, H. S., Jr. Science 2000, 289, 1337-1340.
(2) Huber, C.; Wa¨chtersha¨user, G. Science 1997, 276, 245-247. Huber,
C.; Wa¨chtersha¨user, G. Science 1998, 281, 670-672.
Previously the best models for this site included the octahedral
complex [Fe(SR)3(PR3)(CN)(CO)]2-, the Fe unit in [{Fe(NS3)-
(3) Levy, M.; Miller, S. L.; Oro, J. J. Mol. EVol. 1999, 49, 165-168 and
references therein.
(9) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.; Fernandez,
V. M. J. Am. Chem. Soc. 2000, 122, 11232-11233.
(10) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc.
1999, 121, 9736-9737.
(4) Walters, M. A.; Dewan, J. C. Inorg. Chem. 1986, 25, 4889-4893.
(5) Selected IR spectra in MeCN for 1-5 (cm-1). 1a: νCN 2103(w), 2094-
(w), 2075(vw); νCO 2007(s), 1976(sh) 1953(s). 2: νCN 2103 (s); νCO 1999 (s).
3: νCN 2080(w), 2075(vw); νCO 1897(s). 4: νCN 2104(vw), 2096(w); νCO 2006-
(s), 1949(s). 5: νCN 2196(s), 2159(s); νCO 2005(s), 1949(s).
(6) (a) Taka´cs, J.; Marko´, L. Trans. Met. Chem. 1984, 9, 10-12. (b) Taka´cs,
J.; Marko´, L.; Pa´rka´nyi, L. J. Organomet. Chem. 1989, 361, 109-116. (c)
Taka´cs, J.; Soo´s, E.; Nagy-Magos, Z.; Marko´, L.; Gervasio, G.; Hoffmann,
T. Inorg. Chim. Acta 1989, 166, 39-46.
(11) (a) Sellmann, D.; Kleine-Kleffmann, U.; Zapf, L.; Huttner, G.; Zsolnai,
L. J. Organomet. Chem. 1984, 263, 321-331. (b) Ghilardi, C. A.; Laschi, F.;
Midollini, S.; Orlandini, A.; Scapacci, G.; Zanello, P. J. Chem. Soc., Dalton
Trans. 1995, 531-540. (c) Touchard, D.; Fillaut, J.-L.; Khasnis, D. V.;
Dixneuf, P. H.; Mealli, C.; Masi, D.; Toupet, L. Organometallics 1988, 7,
67-75.
(7) Nagy-Magos, Z.; Marko´, L.; Szaka´cs-Schmidt, A.; Gervasio, G.;
Belluso, E.; Kettle, S. F. Bull. Soc. Chim. Belg., 1991, 100, 445-458.
(8) Reihlen, H.; Friedolsheim, A. v.; Ostwald, W. Justus Liebigs Ann. Chem.
1928, 465, 72-96.
(12) (a) Volbeda, A.; Garcin, E.; Piras, C.; de Lacey, A. L.; Fernandez, V.
M.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. J. Am. Chem. Soc.
1996, 118, 12989-12996. (b) Fontecilla-Camps, J. C.; Ragsdale, S. W. AdV.
Inorg. Chem. 1999, 47, 283-333.
10.1021/ja015948n CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/21/2001

6934 J. Am. Chem. Soc., Vol. 123, No. 28, 2001
Communications to the Editor
Figure 2. Structure of the dianion in (Et4N)2[Fe(S2C6H4)(CN)2(CO)] (3)
with thermal ellipsoids set at the 50% probability level. Selected bond
distances (Å) and angles (deg): Fe-S1, 2.1987(6); Fe-S2, 2.2081(6);
Fe-C1, 1.922(2); Fe-C2, 1.926(2); Fe-C3, 1.694(2); C1-Fe-C2,
88.24(8); C1-Fe-C3, 95.93(9); C2-Fe-C3, 93.85(9); S1-Fe-S2,
89.49(2).
Figure 3. Molecular structure of the dianion of trans-(PPh4)2[Fe(CN)4-
(CO)2] (2) with thermal ellipsoids drawn at the 50% level. Selected bond
distances (Å) and angles (deg): Fe(1)-C(1), 1.811(7); Fe(1)-C(2), 1.816-
(8); Fe(1)-C(3), 1.948(8); Fe(1)-C(4), 1.952(8); Fe(1)-C(5), 1.934-
(8); Fe(1)-C(6), 1.951(8); C-O(avg), 1.145; C-N(avg), 1.168; C(1)-
Fe(1)-C(2), 175.1(3); Fe(1)-C-O(avg), 176 Fe(1)-C-N(avg), 176;
NC-Fe-CN (avg), 90; NC-Fe-CO(avg), 90.
(CO)2}NiCl(dppe) (NS3 ) N(CH2CH2S)3)], and the cyclopenta-
dienyl derivative [CpFe(CN)2CO)]-.13,14 In 3, the complete
Fe(SR)2(CN)2(CO) microenvironment of the enzyme is replicated.
The IR spectrum for 35 exhibits νCO and νCN which are lower
than those reported for the Fe center in D. gigas (2093, 2083,
1947 cm-1).12 Coordination of a Ni(SR)2 unit to the sulfur atoms
in 3 would, however, decrease electron density at Fe with a
corresponding increase in νCO and νCN to values more similar to
those in the Fe(CN)2(CO) unit in the enzyme. Such a shift in νCO
was observed13b in the conversion of [Fe(NS)3(CO)]- (νCO ) 1885
cm-1) into {Fe(NS3)(CO)2}NiCl(dppe) (νCO ) 1944, 2000 cm-1).
Thus 3 is an obvious precursor to bimetallic NiFe derivatives.
Carbonylation of an MeCN solution of 3 gave cis,cis-[Fe(bdt)-
(CN)2(CO)2]2- (4), while a nitrogen purge reverses this reaction.
The reversible carbonylation of 3 is relevant to the proposed
binding of H2 at Fe in the [NiFe]-hydrogenases. NMR measure-
ments show that 3 reacts with Et4NCN to give [Fe(bdt)(CN)3-
(CO)]3-, although under CO, the tricyanide reverts to 4, demon-
strating that CN-, which is normally considered a potent ligand,
is displacable by CO under mild conditions.
2CN-
2Fe2(S2C6H4)(CO)6
8
[Fe2(S2C6H4)2(CN)2(CO)4]2- + 2Fe0 (1)
2CN-
[Fe2(S2C6H4)2(CN)2(CO)4]2-
8
2[Fe(S2C6H4)(CN)2(CO)2]2- (2)
Finally, the above reactions prompted an investigation on the
influence of cyanide on the carbonylation of iron salts in the
absence of thiolates. The addition of Et4NCN to a MeCN solution
of FeCl2 under an atmosphere of CO rapidly gave rise to a series
of CO adducts, which are under further study. Using four equiv
of cyanide, we obtained ∼50% yield of trans-[Fe(CN)4(CO)2]2-
(2), isolated as its Et4N+ salt. Complex 2 was crystallographically
characterized as the PPh4+ salt (Figure 3).18 The only previously
characterized FeII-CN-CO complex is [Fe(CN)5(CO)]3-, which
is synthesized indirectly from preformed [Fe(CN)5L]3- deriva-
tives.19
Analogous to the preparation of 3, the reaction of t-BuNC,
FeCl2, Na2bdt, and CO gave Fe(bdt)(CO)2(CN-t-Bu)2 (5). It is
interesting that neutral 5 is relatively stable in the 18 e- form,
i.e., it does not decarbonylate. In contrast dianion 4 readily loses
CO to give the 16 e- derivative. This finding suggests that
negative charge significantly enhances S-to-Fe π donation in 3,
an issue that will be subjected to further computational analysis.
The key role of cyanide is attributable to its stabilization of
low-spin Fe(II), which then enables the binding of CO, which
then opens the door to reactivity of biosynthetic significance.
Compound 3 also forms in excellent yield (based on bdt2-
)
Acknowledgment. This research was supported by the National
Institutes of Health and the Department of Energy.
from the reaction of Fe2(bdt)(CO)6 (6) with 2 equiv of Et4NCN.
This transformation is unexpected based on previous studies on
the reaction of CN- with Fe2(SR)2(CO)6 (R ) alkyl, CnH2n,
C6H5).10,15,16 This disproportionative transformation occurs via
Supporting Information Available: Tables of atomic coordinates,
selected bond distances and angles, thermal parameters, selected spec-
troscopic, and preparative details (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
the intermediacy of [Fe(bdt)(CN)(CO)2]2 ,
2- 17 which formed when
6 was treated with 1 equiv of Et4NCN (eqs 1 and 2).
JA015948N
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(18) The structure of 2 has also been recently reported. See: Jiang, J.;
Koch, S. A. Abstracts of Papers; 221st National Meeting of the American
Chemical Society, San Diego, CA, April 1-5, 2001; American Chemical
Society: Washington, DC, 2001; Abstract INOR 525.
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1243-1253.
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